It is well known that impaired condition of the electric insulation with leakage of current may lead not only to improper operation of the entire electric system but to more serious consequences such as injury and death of people. An example of this is the crash of Boeing 747 off Long Island in July, 1996. Another crash occurred in 1990 with Boeing 737 on the tarmac in Manila. Investigation showed that in both cases a possible reason of the crash was damaged wire insulation which could allow a surge of a high voltage into the fuel tank (see information from Internet: "FAA Targets Wiring of 737s. Action Stems from Flight 800 Explosion" by Sylvia Adcock, Staff Writer).
Therefore constant monitoring of conditions of electric insulation in electric systems is an extremely important issue.
Methods for measuring electric insulation resistance in a "cold" (non-energized) state of electric systems are known. Such methods possess a plurality of disadvantages the main of which are the following: 1) in order to control electric insulation resistance, the electric system or its portion which is subject to control must be de-energized.; 2) inaccuracy in evaluating the results of the aforementioned control; this inaccuracy resulting from the fact that the insulation resistance characteristics are measured in a non-operative state of the electric system; 3) inability of urgent measures for preventing accelerated wear and damage of the insulation.
In view of the above, obtaining reliable data about the current condition of the electric insulation in the entire electric system or in its part under live or energized conditions is an extremely important issue (see, e.g.,: Meetings of SIGRE WG 15/33-08 "Insulation Monitoring and Life Estimation", Aug. 26-28, 1996, Paris; Sep. 5, 1997, Lake George, N.J.).
Attempts have been made to develop methods and apparatus for obtaining information on the state of the electric insulation under live conditions. For example, U.S. Pat. No. 4,301,399 issued in 1981 to H. Miller describes an electric cable having means for monitoring conditions of the electric insulation between various layers of the cable. This is achieved by interposing an electricaly conductive integrity monitoring layer between the two insulating layers so that a first one of the insulating layers is disposed between the conductive members and the conductive layer, and a second one of the insulating layers is disposed between the conductive layer and the environment surrounding the device. The system monitors the impedance of the first layer between the conductive members and the conductive layer, as well as the impedance of the second layer between the conductive layer and the surrounding environment.
A disadvantage of the system and method of U.S. Pat. No. 4,301,399 is that the cable design must incorporate additional elements for monitoring conditions of cable insulation. Another disadvantage is that the method and system of U.S. Pat. No. 4,301,399 is applicable only to electric cables and is not applicable to other electric systems and devices such as electric motors or multiple-component electric systems.
U.S. Pat. No. 5,287,062 issued in 1994 to C. Pellgrin et al. discloses a device for monitoring direct current (DC) power supply systems. In this device, an alternative current (AC) signal is "injected" into a DC electrical network for performing insulation measurement in a processing unit. The AC signal is measured in a mixed bridge having a first resistive dividing bridge connected in parallel with a second capacitive dividing bridge between the line and the ground. The intermediate output points of the first and second bridges are joined together and connected to an input of the processing unit. The impedance and the dividing ratio of the two bridges cause the DC voltage measured at said input to be low in relation to the value of the AC voltage measured at the output of said mixed bridge.
A disadvantage of the devices of the type described in U.S. Pat. No. 5,287,062 is that they are complicated in structure because they require an additional power source of a predetermined frequency. Furthermore, these devices are not applicable to AC systems or DC systems which do not allow the presence of an AC signal.
U.S. Pat. No. 5,117,191 issued in 1992 to A. Saigo, et al. describes an apparatus for monitoring degradation of insulation in electrical installations. In addition to measuring units, this apparatus contains phase comparison units, phase detection units, a pulse forming circuit, a timing setting circuit, etc. As a result, the monitoring system is very complicated and therefore expensive to manufacture and unreliable in operation.
U.S. Pat. No. 4,896,115 issued in 1990 to M. Gerin describes an electrical network insulation monitoring and measuring device. This device is also complicated in structure because it contains sources of different frequencies (10 Hz and 1 kHz) as well as several converters for processing the signals of different frequencies.
U.S. Pat. No. 5,032,795 issued in 1991 to J. Asars, et al. employs an ion source that generates an ion cloud and causes this cloud to displace along the airplane cable. The device also utilizes signals of different frequencies for detecting insulation damages. It possesses the same disadvantages as the earlier mentioned devices and systems, i.e., it is complicated in structure and has a limited application.